Where it is desired to transmit data from a master station to a number of slave stations located within a comparatively short distance in the order of several kilometers from the master station, through a pair of signal lines connected therebetween, for example, as in a vending system of a hotel or the like, in which data of sold articles are transmitted from automatic vending machines equipped in guest rooms to a control unit in the hotel front through signal lines connected therebetween, it is required that each slave station has a data transmission device simple in structure, and transmits and receives data to and from the master station with a high reliability, independently of an external noise and without breaking or losing data, in the event of a power failure, thus ensuring the minimum data transmission function, irrespective of any accident.
On the other hand, a variety of highly reliable data transmission systems have hitherto been proposed. For example, a frequency modulation type data transmission system is known as being highly stable against an external noise. Such a frequency modulation type data transmission system, however, requires complicated circuit arrangement, incuring an increased manufacturing cost. From the economical viewpoint, the frequency modulation type data transmission system failed to find applications to a small scale, data transmission system, such as a hotel vending system described above. In general, for a small scale data transmission system such as a hotel vending system, in which only a small amount of data are transmitted at a comparatively low transmission rate, it is essential and important that circuit arrangements are less costly and easy to install, but perform transmission of data without decreasing reliability.
It is usual, in a wire data transmission system, to set an impedance at a receiving terminal at a high level, in order to avoid an adverse influence due to the lowered impedance in transmission lines. The transmission system having such a high impedance, however, is disadvantageously susceptible to an external noise, because a line voltage is likely to fluctuate even by a small noise energy. Furthermore, it is difficult, in the data transmission system having a high impedance, to exchange energy between the master station which is the center of the data transmission and a number of slave stations which are terminals of the data transmission with the result that in the event that a power source voltage in a certain slave station becomes zero due to a power failure, supply of a power source voltage from the master station to the slave station is interrupted, resulting in the failure of the transmitting and receiving functions in the slave station. With a view to maintaining the transmitting and receiving functions on the slave station side, irrespective of the loss of a power source voltage, an attempt has been proposed, for providing an interruption-free power source for the slave station incurring a greatly increased cost of equipment.
Since a signal line in the wire data transmission system extends over several kilometers, various surge voltages, such as a surge voltage due to lightning induction or a surge voltage due to induction from power supply wiring, tend to be introduced into the signal line. To cope with introduction of such surge voltages into the signal line, it is customary to interpose a surge-suppressing circuit element between the signal lines, as will be described later. The wire data transmission system of the type accordingly involves a risk that the surge-suppressing circuit element interposed between the signal lines remains short-circuited due to a surge voltage introduced into the signal lines, or the signal lines remain short-circuited due to the break of an output transistor in the master station or the break of a transmitting transistor in the slave station, with a likelihood of the impossibility of transmission of data. In order to remove the short circuit failure between signal lines, terminal circuit elements in the respective slave stations must be checked up to detect a point of failure. The checking-up of the terminal circuit elements in the respective slave stations in the wire data transmission system requires much labor and is greatly time-consuming, particularly in a vending system of a hotel, because signal lines are embedded in the walls of the building, a number of slave stations are distributed in guest rooms, respectively, and terminal circuit elements in an individual slave station must be electrically separated, for checking-up, from the signal lines sequentially. With the conventional wire data transmission system of the type, there has been a drawback in that the immediate removal of the short circuit failure between the signal lines for the assurance of the reliability in performance has been difficult. Another drawback encountered with the prior art wire data transmission system is that the wiring for laying out a pair of transmission lines in the wall of a building must be conducted with the possibly deepest care so that the polarities of these transmission lines are located correctly with respect to a DC signal voltage, for example, in a manner that a polarity of one wire is discriminated from that of the other by color. Nevertheless, the conventional wire data transmission system has occasionally suffered from accidents due to miswiring. In the case where diametrically small transmission lines are used for transmission of data over a long distance, for the simplicity in circuit arrangement of the transmission system, an increased attenuation of information signals due to a transmission lines resistance is bound to result. To avoid this, the wiring operation must be performed, taking the lowering of a signal voltage into consideration, and at the same time, the operational conditions of signal receiving circuit equipments in the respective slave stations must be individually adjusted to meet the lowered signal voltage. Placement and maintenance, of such a transmission system are time-consuming and require much labor.